The disclosure relates to gas turbine engines. More particularly, the disclosure relates to tip repairs of turbine blades of such engines.
In many gas turbine engines, turbine section blades have abrasive tips for interfacing with abradable coatings on the inner diameter (ID) surface of blade outer air seals (BOAS) surrounding said blades. A typical turbine blade comprises a metallic substrate (e.g., nickel-based superalloy or cobalt-based superalloy) shaped to form an attachment root (for attaching to a disk) and an airfoil. The exemplary blades further typically include a platform between the attachment root and airfoil. Blade substrates may be cast and machined and may feature internal cooling passageways for receiving a cooling airflow. The cooling passages may have one or more inlets along the attachment root (e.g., at an inner diameter (ID) surface thereof) and many outlets distributed along the airfoil. External surfaces of the blade exposed to the gaspath may bear thermal barrier coatings (TBC). The blade tip (blade airfoil tip) may, however, bear an abrasive coating for interfacing with an abradable coating of the adjacent BOAS stage. An exemplary abrasive coating comprises an abrasive (e.g., cubic boron nitride (CBN)) in a metallic matrix (e.g., nickel alloy). In an exemplary method of manufacture, the abrasive and matrix are applied via electroplating.
In an exemplary specific method of original blade manufacture, a precursor of the substrate is cast (e.g., investment cast). In one or more machining stages, the substrate is machined leaving the airfoil overlong. In one example, it is overlong by 0.010 inch (0.25 millimeter). The length is measured relative to a datum such as a datum of a fir tree attachment root. The same datum provides the primary datum for subsequent processing discussed below. After this initial machining, the blade may be masked both interior and exterior in the vicinity of the tip. The blade tip may then be machined down to the final reference length through the substrate.
The as-cast blade substrate may include a squealer pocket along the tip. The machining may leave the pocket having a desired depth. Outlet passageways from trunks of the cooling passageway system may extend into a base of the squealer pocket.
Thereafter, the abrasive coating may be applied. The exemplary application method involves first applying a base layer and then applying the matrix and abrasive. The exemplary base layer comprises a nickel alloy (e.g., nickel-chromium-aluminum alloy) electroplate applied to a nominal base layer thickness.
Exemplary nominal base layer thickness is 0.0025 inch (0.0635 millimeter). Exemplary base layer application involves an initial flash of nickel followed by application of the remainder. This can provide enhanced adhesion of the base layer relative to directly applying the base layer alloy to the substrate. Eventually, due to diffusion with the nickel alloy, the very thin nickel flash layer will cease to be distinct.
After base layer application, the abrasive and matrix may be applied (e.g., via electroplating) to a desired abrasive layer thickness. Exemplary matrix is a nickel alloy (e.g., nickel-chromium-aluminum alloy which may be the same as that used for the base layer). An exemplary abrasive layer thickness is 0.0055 inch (0.14 millimeter). After such matrix application and any demasking/remasking, the thermal barrier coating may be applied via conventional means. Exemplary thermal barrier coating systems comprise a metallic bondcoat applied directly to the substrate and a ceramic thermal barrier coat applied to the bondcoat.
As the blade is used in engine operation, the tip will encounter wear and damage. Exemplary damage includes cracking and oxidation. A number of prior art tip repair techniques have been proposed and/or practiced. In one typical such technique, the blades are removed from the engine and repaired at an industrial mass production scale. Two basic such repairs are known: repairs using a preform; and repairs using in situ weld buildups.
Preform repairs involve machining off a substantial length of blade potentially even penetrating into the trunks of the internal passageways. A preformed replacement tip is then applied via techniques such as diffusion brazing. The tip may include a surface for reforming features such as the internal passageways and the squealer tip pocket. After attachment, finish machining may machine the preform down to the original blade substrate final reference length and abrasive coating may be applied in the same way as in the original manufacture.
In situ weld build-up repairs may similarly involve building up to a slight oversize/overlength followed by machining down to the original blade substrate final reference length and then duplicating the original tip coating. However, compared with use of preforms, the buildup will typically involve a much smaller degree of material removal from the preexisting substrate.
Each of the repair techniques has disadvantages. Preforms present economy of scale issues. After the first repair using a preform, a subsequent repair will require a larger preform and so on. Weld build-up repairs can place substantial stress on remaining substrate material. Even if the initial machining removes crack material and oxidation, the welding process itself may cause cracks in remaining substrate.
Weld build-up repair also has limited repeatability. The deep machining to remove damaged material in an original blade starts a cascade. The first weld repair damages material substantially below the original machined surface of the substrate. This requires that each subsequent weld repair have a relatively large further penetration into the substrate.